Retrofit for power generation system

ABSTRACT

A method of retrofitting a power generation system includes modifying a pre-existing power generation system that includes a combustor and a steam-based cycle to include a super-critical carbon dioxide-based Brayton cycle that is directly coupled through the combustor. The steam-based cycle is converted into a steam-based Rankine cycle that is in thermal-receiving communication with the super-critical carbon dioxide-based Brayton cycle.

BACKGROUND

This disclosure relates to power plants for generating electricity.

Existing coal-fired power plants that have been in operation for many years, such as supercritical pulverized coal plants, typically suffer from high carbon dioxide emissions. One approach to reduce carbon dioxide emissions is to outfit an existing plant with a post-combustion device, such as a chilled ammonia or hindered amine device, to capture carbon dioxide from combustion exhaust. Although such devices are effective in reducing net carbon dioxide emissions, the devices typically debit overall plant efficiency and thus increase levelized cost of energy.

More recently, there have been proposals to regulate carbon dioxide emissions by capping emissions per unit of electricity produced. Because post-combustion devices debit plant efficiency, the carbon dioxide emissions per unit of generated electricity increases. Therefore, existing plants are ill-equipped to meet such regulations and are faced with the possibility of forced retirement.

SUMMARY

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

FIG. 1 is a schematic view of a pre-existing power generation system.

FIG. 2 is a schematic view of a retrofit power generation system based upon the pre-existing power generation system of FIG. 1.

FIG. 3 is a schematic view of another example retrofit power generation system based upon the pre-existing power generation system of FIG. 1.

FIG. 4 is a schematic view of another example pre-existing power generation system.

FIG. 5 is a schematic view of a retrofit power generation system based upon the pre-existing power generation system of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a schematic view of selected portions of a pre-existing power generation system 20 (“system 20”). The term “pre-existing” generally refers to the system 20 having been in operation for its intended use for some period of time. As disclosed herein, as an alternative to retiring the system 20, the system 20 can be retrofitted with new, more efficient hardware, while retaining at least some of the pre-existing hardware of the system 20, to produce more power per unit of coal or fuel input. As examples, a retrofit system as disclosed herein is expected to achieve 5-10% increase in overall net thermal efficiency, 10-30% lower carbon dioxide emissions, up to 25% reduction in levelized cost of energy and the ability to meet proposed regulations with regard to efficiency and emissions per unit of electricity produced.

The system 20 includes a combustor 22, such as a coal-fired boiler, which receives an input coal feed 24 a and an input oxidant feed 24 b (e.g., air) that generate heat within the combustor 22. A steam-based cycle 26 (power cycle) absorbs heat from the combustor 22 to generate electricity. The steam-based cycle 26 includes a first turbine 28, a second turbine 30 and third turbine 32. The turbines 28/30/32 are mounted on a shaft 34, which is coupled to drive a generator 36. The third turbine 32 is in communication with a condenser 38, which is connected in circuit to the combustor 22. The combustor 22, turbines 28/30/32 and condenser 38 are connected within a closed loop, working fluid circuit 40. For example, the working fluid circuit 40 includes steel tubes that convey water, steam or both between the combustor 22, turbines 28/30/32 and condenser 38, as generally indicated by the arrows in the working fluid circuit 40.

In operation, liquid water is discharged from the condenser 38 into the combustor 22. The combustor 22 generally operates in a temperature regime of less than 700° F./371° C. and pressure of less than 3000 pounds per square inch/20.5 megapascals due to the limits of the materials of the working fluid circuit 40 and the turbines 28/30/32. The water absorbs heat within the combustor 22 and turns to steam. The steam is then expanded over the first turbine 28. The expanded steam from the first turbine 28 is circulated back through the combustor 22 for a reheat. The reheated steam is then expanded over the second turbine 30 and then the third turbine 32. The expanded steam from the third turbine 32 is condensed in the condenser 38 prior to circulation into the combustor 22 for another thermodynamic cycle.

In this example, the system 20 utilizes relatively inefficient technology. For example, the tubes of the working fluid circuit 40 and components of the turbines 28/30/32 are made of steel. In that regard, the working fluid circuit 40 and turbines 28/30/32 have a maximum operating temperature to which the materials of these components can be exposed. For example, the temperature in the combustor 22 is controlled using a water quench or the like to ensure that actual operating temperatures of the steam do not exceed the maximum operating temperature limit of the materials of the working fluid circuit 40 and the turbines 28/30/32. Overall, the operating efficiency of the system 20 is limited by the maximum allowed temperature in the combustor 22 and steam-based cycle 26. Thus, even if carbon dioxide is captured from an exhaust 42 of the combustor 22, the system 20 as-is has only limited ability to improve carbon dioxide emissions per unit of generated electricity and levelized cost of energy.

As will be appreciated from FIG. 2, the system 20 of FIG. 1 has been retrofitted with efficiency enhancements to produce a retrofitted power generation system 20′ (retrofit system 20′). In this disclosure, the term “retrofit” or variations thereof may be used to refer to an individual hardware component or to a system, for example. When used with reference to an individual hardware component for use in a system, the term indicates that the component was not part of the operable initial or prior system and is not a mere replacement in kind of a like component of the operable initial or prior system. When used with reference to a system, the term indicates that the system includes at least some pre-existing hardware components and at least one added hardware component that was not part of the operable initial or prior system and is not a mere replacement in kind of a like component of the operable initial or prior system. The modifying terms “pre-existing” and “retrofit” as used herein thus indicate a physical distinction between components and/or systems.

In this example, the retrofit system 20′ utilizes a portion of the pre-existing hardware of the system 20, including the pre-existing combustor 22, the pre-existing turbines 28/30/32 and the pre-existing condenser 38. However, the working fluid circuit 40 is replaced with a second (retrofit) working fluid circuit 50 that is directly coupled through the combustor 22 and the retrofit system 20′ includes at least one additional, retrofit turbine 52 mounted on the shaft 34. Although only one retrofit turbine 52 is shown, it is to be understood that additional retrofit turbines 52 could be used.

In the retrofit system 20′, the retrofit turbine 52, the combustor 22, the turbines 28/30/32 and condenser 38 are connected within the second working fluid circuit 50. For example, the second working fluid circuit 50 includes superalloy tubes that convey water, steam or both between the combustor 22, retrofit turbine 52, turbines 28/30/32 and the condenser 38, as generally indicated by the arrows in the second working fluid circuit 50. A “superalloy” as used herein refers to a nickel-based, cobalt-based or nickel-iron-based alloy.

In operation, liquid water is discharged from the condenser 38 into the combustor 22. The water absorbs heat within the combustor 22 and turns to steam. The steam is then expanded over the retrofit turbine 52. The expanded steam from the retrofit turbine 52 is then serially expanded over the first turbine 28, the second turbine 30 and the third turbine 32. The expanded steam from the third turbine 32 is then condensed in the condenser 38 prior to being circulated to the combustor 22 for another thermodynamic cycle.

The retrofit system 20′ has enhanced efficiency in comparison with the system 20 with regard to carbon dioxide emissions per unit of electricity generated. For example, the tubes of the second working fluid circuit 50 and components of the retrofit turbine 52 are made of superalloy materials. In that regard, the second working fluid circuit 50 and retrofit turbine 52 have a second maximum operating temperature that is greater than the maximum operating temperature of the prior working fluid circuit 40 and turbines 28/30/32 that include steel or other lower melting point materials. The second working fluid circuit 50 can thus be routed through a hotter portion 22 a of the combustor 22 than the prior working fluid circuit 40, or the combustor 22 can be operated at a higher temperature to generate higher temperature steam. For example, the combustor 22 operates in a temperature regime of up to 1300° F./705° C. and pressure of up to 6000 pounds per square inch/41 megapascals. Once the higher temperature steam is expanded over the retrofit turbine 52, the steam cools to a temperature that is within the maximum operating temperature of the turbines 28/30/32. Thus, the retrofit system 20′ can be operated at higher, more efficient temperatures to improve carbon dioxide emissions per unit of generated electricity and to reduce levelized cost of energy.

As will be appreciated from another example of a retrofit in FIG. 3, the system 20 of FIG. 1 is retrofitted with efficiency enhancements to produce a retrofitted power generation system 20″ (retrofit system 20″). In this example, the system 20 has been retrofitted with a super-critical carbon dioxide-based Brayton cycle 54 to enhance efficiency. The retrofit system 20″ utilizes a portion of the pre-existing hardware of the system 20, including the pre-existing combustor 22, pre-existing turbine 32 and pre-existing condenser 38, The working fluid circuit 40 is replaced with a second (retrofit) working fluid circuit 50′ that extends through the combustor 22. The retrofit system 20″ also includes at least one additional, retrofit turbine 52′ mounted on the shaft 34.

The super-critical carbon dioxide-based Brayton cycle 54 is thermally coupled through the combustor 22 and the prior steam-based cycle 26 is converted to a steam-based Rankine cycle 26′ that is in thermal-receiving communication with the super-critical carbon dioxide-based Brayton cycle 54.

As an example of the retrofit, the prior steel tubes of the working fluid circuit 40 are removed, including removal from the combustor 22. Superalloy tubes of the second working fluid circuit 50′ are added and are directly coupled through the combustor 22, The addition of the super-critical carbon dioxide-based Brayton cycle 54 includes adding a retrofit compressor 56, a retrofit first turbine 58 and a retrofit second turbine 60. The prior steam-based cycle 26 is modified to add a retrofit heat exchanger 62 for thermal communication between the super-critical carbon dioxide-based Brayton cycle 54 and the steam-based Rankine cycle 26′. The retrofit compressor 56, the retrofit first turbine 58, the retrofit second turbine 60 and the pre-existing turbine 32 are mounted on the common shaft 34 to drive the generator 36. The retrofit first turbine 58 and the retrofit second turbine 60 each includes a rotor having a disk 66 and a plurality of blades 68 mounted on the disk 66.

In operation, a working fluid, such as carbon dioxide or a carbon dioxide-containing mixture (e.g., with helium), in the second working fluid circuit 50′ absorbs heat within the combustor 22 and is then expanded over the retrofit first turbine 58. The expanded working fluid is then circulated back into the combustor 22 for a reheat. The reheated working fluid is then expanded over the retrofit second turbine 60 and then circulated to the retrofit heat exchanger 62. The working fluid in the retrofit heat exchanger 62 heats water within the steam-based Rankine cycle 26′. The working fluid is then pressurized in the retrofit compressor 56 prior to circulating to the combustor 22 for another thermodynamic cycle. The heated steam from the heat exchanger 62 expands over the pre-existing turbine 32 and then circulates to the condenser 38 for another thermodynamic cycle.

The retrofit system 20″ has enhanced efficiency in comparison with the system 20 with regard to carbon dioxide emissions per unit of electricity generated. For example, the tubes of the second working fluid circuit 50′ and the disks 66 and blades 68 of the retrofit turbines 58/60 are made of superalloy materials. In that regard, the second working fluid circuit 50′ and retrofit turbines 58/60 have a second maximum operating temperature that is greater than the maximum operating temperature of the prior working fluid circuit 40 and turbines 28/30/32 that include steel materials. The second working fluid circuit 50′ can thus be routed through a hotter portion 22 a of the combustor 22 than the prior working fluid circuit 40, or the combustor 22 can be operated at a higher temperature to generate higher temperature working fluid. For example, the combustor 22 operates in a temperature regime of up to 1300° F./705° C. and pressure of up to 6000 pounds per square inch/41 megapascals. Thus, the retrofit system 20″ can be operated at higher, more efficient temperatures to improve carbon dioxide emissions per unit of generated electricity and to reduce levelized cost of energy.

FIG. 4 illustrates another example pre-existing power generation system 120. In this example, the pre-existing power generation system 120 includes a combustor 1 which in this example is a fluidized bed reactor that receives a coal feed 124 and an adsorbent feed 125, such as limestone, which facilitates the reaction within a fluidized bed 122 a. Alternatively, the combustor 122 can be a coal-fired boiler that is then replaced with a retrofit fluidized bed reactor, coal feed 124 and adsorbent feed 125.

A steam--based cycle 126 absorbs heat from the combustor 122 to generate electricity. The steam-based cycle 126 includes a heat exchanger 170 and a turbine 132 that is mounted on a shaft 134. The turbine 132 is coupled through the shaft 134 to drive a generator 136. The heat exchanger 170 is in communication with circuit 140, which receives a hot exhaust stream from the combustor 122 as generally indicated by the arrows in the circuit 140. Similar to the system 20, in the system 120 the tubes of the circuit 140 and components of the turbine 132 are made of steel and have a maximum operating temperature.

In operation, the combustor 122 produces a hot exhaust stream that is discharged through circuit 140 to the heat exchanger 170. The hot exhaust stream heats water in the heat exchanger 170 to produce steam. The hot exhaust stream may then be recycled downstream from the heat exchanger 170 such that at least a portion of the product stream, such as carbon dioxide, is fed back into the combustor 122. The steam in the steam-based cycle 126 expands over the turbine 132 to drive the generator 136.

As will be appreciated from FIG. 5, the system 120 of FIG. 4 has been retrofit with efficiency enhancements to produce a retrofitted power generation system 120′ (retrofit system 120′). In this example, the retrofit system 120′ has been retrofitted with a super-critical carbon dioxide-based Brayton cycle 154 to enhance efficiency. The retrofit system 120 utilizes a portion of the pre-existing hardware of the system 120, including the pre-existing turbine 132 and pre-existing heat exchanger 170. A second (retrofit) working fluid circuit 150′ that extends through the combustor 122 is added. The retrofit system 120″ also includes at least one additional, retrofit turbine 152 mounted on the shaft 134.

The super-critical carbon dioxide-based Brayton cycle 154 is thermally coupled through the combustor 122 and the prior steam-based cycle 126 is converted to a steam-based Rankine cycle 126′ that is in thermal-receiving communication with the super-critical carbon dioxide-based Brayton cycle 154.

As an example of the retrofit, superalloy tubes of the second working fluid circuit 150′ are added and are directly coupled through the combustor 122. The addition of the super-critical carbon dioxide-based Brayton cycle 154 includes adding a retrofit compressor 156, a retrofit first turbine 158 and a retrofit second turbine 160. The prior steam--based cycle 126 is modified to add a retrofit heat exchanger 162 for thermal communication between the super-critical carbon dioxide-based Brayton cycle 154 and the steam-based Rankine cycle 126. The retrofit compressor 156, the retrofit first turbine 158, the retrofit second turbine 160 and the pre-existing turbine 132 are mounted on the common shaft 134 to drive the generator 136. The retrofit first turbine 158 and the retrofit second turbine 160 each includes a rotor having a disk 166 and a plurality of blades 168 mounted on the disk 166.

In operation, a working fluid, such as carbon dioxide or a carbon dioxide-containing mixture (e.g., with helium), in the second working fluid circuit 150′ absorbs heat within the fluidized-bed 122 a and is then expanded over the retrofit first turbine 158. The expanded working fluid is then circulated back into the combustor 122 for a reheat. The reheated working fluid expands over the retrofit second turbine 160 and then circulates to the retrofit heat exchanger 162. The working fluid in the retrofit heat exchanger 162 heats water within the steam-based Rankine cycle 126′. The working fluid is then pressurized in the retrofit compressor 156 prior to circulating to the combustor 122 for another thermodynamic cycle. The heated steam from the heat exchanger 162 expands over the pre-existing turbine 132 and then circulates to a condenser 138 for another thermodynamic cycle.

The retrofit system 120′ has enhanced efficiency in comparison with the system 120 with regard to carbon dioxide emissions per unit of electricity generated. For example, the tubes of the second working fluid circuit 150′ and the disks 166 and blades 168 of the retrofit turbines 158/160 are made of superalloy materials. Thus, the second working fluid circuit 150′ and retrofit turbines 158/160 have a second maximum operating temperature that is greater than the maximum operating temperature of the circuit 140 and turbine 132 that include steel materials. The second working fluid circuit 150′ can thus be routed through the fluidized-bed 122 a, or the combustor 122 can be operated at a higher temperature. For example, the combustor 122 operates in a temperature regime of up to 1300° F./705° C. and pressure of up to 6000 pounds per square inch/41 megapascals. Thus, the retrofit system 120′ can be operated more efficiently to improve carbon dioxide emissions per unit of generated electricity and to reduce levelized cost of energy.

Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.

The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims. 

What is claimed is:
 1. A method of retrofitting a power generation system, the method comprising: in a pre-existing power generation system including a combustor and a steam-based cycle, modifying the pre-existing power generation system to include a super-critical carbon dioxide-based Brayton cycle that is directly coupled through the combustor; and converting the steam-based cycle into a steam-based Rankine cycle that is in thermal-receiving communication with the super-critical carbon dioxide-based Brayton cycle.
 2. The method as recited in claim 1, wherein: the super-critical carbon dioxide-based Brayton cycle includes at least one turbine and the steam-based cycle includes at least one turbine, and mounting the at least one turbine of the super-critical carbon dioxide-based Brayton cycle and the at least one turbine of the steam-based cycle on a common shaft to drive a generator.
 3. The method as recited in claim 1, wherein the modifying includes providing superalloy tubes that extend through the combustor.
 4. The method as recited in claim 3, including locating the superalloy tubes through a first portion of the combustor that is hotter than a second, different portion of the combustor through which tubes of the steam-based cycle extended prior to the modification.
 5. The method as recited in claim 1, wherein the converting includes removing tubes of the steam-based cycle from the combustor.
 6. The method as recited in claim 1, wherein the converting includes connecting a heat exchanger in communication with the super-critical carbon dioxide-based Brayton cycle and the steam-based Rankine cycle.
 7. The method as recited in claim 1, wherein the combustor is a fluidized-bed reactor.
 8. The method as recited in claim 7, including thermally coupling the super-critical carbon dioxide-based Brayton cycle directly through the fluidized-bed reactor.
 9. A method of retrofitting a power generation system, the method comprising: providing a pre-existing power generation system comprising a combustor and a steam-based cycle, the steam-based cycle including a first working fluid circuit extending through the combustor and at least one turbine in fluid communication with the first working fluid circuit, the at least one turbine being mounted on a shaft that is coupled to drive a generator, the first working fluid circuit and the at least one turbine defining a first maximum operating temperature; replacing the first working fluid circuit with a second working fluid circuit extending through the combustor; and adding at least one additional turbine mounted on the shaft, the at least one additional turbine being in fluid communication with the second working fluid circuit and the at least one turbine, the second working fluid circuit and the at least one additional turbine defining a second maximum operating temperature that is greater than the first maximum operating temperature.
 10. The method as recited in claim 9, wherein the adding of the at least one additional turbine includes arranging the at least one additional turbine upstream of the at least one turbine such that the at least one turbine is in flow-receiving communication with the at least one additional turbine.
 11. The method as recited in claim 9, wherein the first working fluid circuit includes steel tubes and the second working fluid circuit includes superalloy tubes.
 12. The method as recited in claim 9, wherein the at least one additional turbine includes superalloy blades.
 13. A retro-fitted power generation system comprising: a combustor; a working fluid circuit extending through the combustor; at least one pre-existing turbine having a first maximum operating temperature; at least one retrofit turbine arranged in fluid communication with the working fluid circuit and the at least one pre-existing turbine, the at least one retrofit turbine having a second, greater maximum operating temperature.
 14. The system as recited in claim 13, wherein the combustor is selected from the group consisting of a coal-fired boiler and a fluidized bed reactor.
 15. The system as recited in claim 13, wherein the working fluid circuit includes superalloy tubes extending through the combustor.
 16. The system as recited in claim 13, wherein the at least one pre-existing turbine includes steel and the retrofit turbine includes a superalloy material.
 17. The system as recited in claim 13, wherein the at least one pre-existing turbine and the retrofit turbine are mounted on a common shaft.
 18. The system as recited in claim 13, wherein the at least one retrofit turbine is arranged upstream of the at least one pre-existing turbine such that the at least one pre-existing turbine is in flow-receiving communication with the at least one retrofit turbine. 